Scientists believe that early in its history, Mars may have had lots of liquid water, a thicker atmosphere and more active volcanism. It's thought that some 3.5 million years ago, Earth and Mars may even have had similar ecological environments.

Turning Up the Heat on Mars

The same greenhouse gasses that pollute the Earth would be key to warming Mars. The following options are among the methods being considered. For safety reasons, the settlers may need to leave the planet's surface for a time.

Orbital Mirrors
Scientists have considered placing Mylar disks, with a diameter of 155 miles and a weight of 200,000 tons, to reflect sunlight onto Mars and heat the surface. The idea is that over many years, the rise in temperature would release greenhouse gasses. The question remains how to get such large mirrors into space, so it's likely they would have to be built there, and not on Earth.

Greenhouse Gas Factories
Scientists contemplate building greenhouse gas-producing factories that generate large quantities of chlorofluorocarbons, methane, and carbon dioxide, which would then trap solar radiation and help build a thicker Martian atmosphere. Whether the material for constructing the factories could be shipped to Mars or made from Martian elements is being debated.

Ammonia-Heavy Asteroids
Scientists have thought about attaching nuclear, thermal-rocket engines to ammonia-heavy asteroids and redirecting the asteroids so that they crash into Mars and release the meteor's ammonia and water. The ammonia could raise Mars' greenhouse gas level. The rocket engines could be able to move the asteroids 3 miles per second, for a period of ten years, before they shut down and allowed the asteroids to coast into Mars.